QSFP28 vs QSFP-DD vs QSFP56, What Are the Differences?

What is QSFP28? 

QSFP28 is a type of QSFP form factor that uses 4 channels (25Gbps per channel) to support data transfer speeds of up to 100Gbps. The QSFP28 form factor is a compact, high-density, and low power consumption 100 Gigabit Ethernet connectivity option for data centers and other high-speed applications. In recent years, shipments of QSFP28 optical transceivers is increasingly rising with the rapid development of higher bandwidth and speed networks.

The common 100G QSFP28 transceivers include 100G SR4, LR4, CWDM4, and PSM4. Their main specifications are shown below.

Besides QSFP28 transceivers, 100G QSFP28 DAC cable is also popular for short-range applications, including QSFP28 to QSFP28 DAC cable, QSFP28 to 4x SFP28 DAC cable. These cables are typically used to connect two QSFP28 ports within a rack or cabinet with a max distance of 10 meters.

What is QSFP56? 

200G optical modules with PAM-4 technology

QSFP56 is a relatively new form factor for data communications, designed to support data rates up to 200Gbps. The QSFP56 transceiver is compliant with the QSFP56 Multi-Source Agreement (MSA). 200G QSFP56 transceiver is an upgraded version of QSFP28.  It uses the same physical specifications as prior QSFP28, the biggest improvement is the modulation technology. QSFP56 upgrades the modulation technology by using PAM-4 instead of NRZ of 100G QSFP28. While NRZ needs 8x25Gbps to reach 200Gbps, PAM4 only needs 4×50Gbps to support 200Gbps, so as to save the optical fiber costs and reduce the link loss. 

The common 200G QSFP56 transceivers include 200G QSFP56 SR4, DR4, FR4, LR4, and ER4. Their main specifications are shown below.

What is QSFP-DD?

The smallest 400G optical transceiver

QSFP-DD means Quad Small Form-factor Pluggable Double Density, also referred to as QSFP56-DD, which is a next-generation, high-density connector interface for high-performance data center applications. It is compliant with the IEEE802.3bs and QSFP-DD MSA. Different from 4-channel QSFP56 transceivers, QSFP-DD uses 8-channel electrical interface, reaching 200G data rates over 8x25Gb/s NRZ, and also supports 400G over 8x50Gb/s PAM-4, which is the smallest 400G optical module. 

Backward compatible with QSFP+, QSFP28, and QSFP56

One of the most significant features of QSFP-DD is backward compatibility. The QSFP-DD connector has the same pinout as the older QSFP connector. It is backward compatible with 40G~200G QSFP type optical modules, including the prior 40G QSFP+, 100G QSFP28, and 200G QSFP56. That is, the existing QSFP+ and QSFP28 transceivers can be inserted into QSFP-DD slots without any adapters. So QSFP-DD is a flexible solution for upgrading current 40G/100G networks to 400G. As a result, QSFP-DD provides a high-performance, high-density, and scalable interface that is compatible with a wide range of existing equipment.

The common QSFP-DD transceivers include 400G QSFP-DD SR8, DR4, FR4, LR4, LR8 and 200G QSFP-DD SR8, PSM8, LR8. Their main specifications are shown below.

Comparison of  QSFP28 vs QSFP-DD vs QSFP56

Differences Quick View 

The following table concludes the main differences of QSFP28, QSFP-DD and QSFP56.

The following will give a detail illustration to their differences.

Differences 1: Data rate and channel count

Data rate is the most fundamental difference among them. QSFP-DD is the max data rate one, as its name implies, double density, that is its channel count has doubled from 4-channel of QSFP28 and QSFP56 to 8 channels. As a result, QSFP-DD supports a max data rate of 400G (8x 50Gb/s) based on PAM4 modulation, and also supports 200G (8x 25Gb/s) based on NRZ modulation. And with 4 channels modulated with NRZ results of 50Gbps/25Gbps per channel, QSFP56 supports 200Gbps whereas QSFP28 supports 100Gbps.

Differences 2: NRZ vs PAM4 Modulation

As mentioned above, QSFP28 and 200G QSFP-DD use NRZ modulation technology while QSFP56 and 400G QSFP-DD use PAM4 technology. Then what are the differences between NRZ and PAM4?

Why we need PAM4

NRZ (Non-Return-to-Zero) is the main modulation technology in non-coherent optical communications. However, with the increasing data rate, such as 200G/400G, NRZ faced difficulties in dispersion, bandwidth, and costs. When the baud rate is above 25G, the influence of dispersion begins to be significant in medium and long-distance transmission. When the electrical-optical conversion bandwidth is more than 60 Gbit/s, the technical bottleneck comes. In terms of cost, under the same bit rate, the baud rate of PAM4 is only half of that of NRZ, which greatly reduces the bandwidth requirements of optical devices, thus reducing the cost of optical devices. As a result, PAM4 modulation technology has become an inevitable development direction.

Challenges of PAM4

PAM4 also has disadvantages, including, 

• The SNR of PAM4 is worse than NRZ at -9.54dB.

• Reflections of PAM4 are 3x worse than NRZ.

• Higher power consumption than NRZ.

• Higher cooling conditions are required.

• More expensive equipment is required.

Since both QSFP56 and QSFP-DD support 200G data rates, factoring in SNR, power consumption, easy deployment, and cost, the 200G QSFP-DD type might be the better choice for the 200G short-range interconnection option.

Conclusion

QSFP28, QSFP-DD, and QSFP56 can be seen as variants of the QSFP form factor type. They have similarities, but of course differ in many ways, different data rates, different channel counts, different modulation types, etc. Although 200G and 400G optical transceivers are not widespread for now, networking is changing fast, 200G and 400G optical transceivers must develop and dominate markets in the near future. Earlier deployment of a 200G/400G network for your data center is necessary. 400G QSFP-DD transceivers are available at QSFPTEK, welcome to consult via sales@qsfptek.com.